Is there life on 'exo-planets'?
There are, we now know, as many planets as there are stars in the night sky - maybe around a hundred billion in our own galaxy (the Milky Way) alone.
It is an astonishing number, and all the more so since we only knew of nine planets in the 1990's, and even that number promptly dropped to eight when Pluto was demoted to dwarf-planet status in 2006.
However if "exo-planets" (planets orbiting distant stars) are commonplace, two-a-penny even, scientists still do not know if any of them support life.
The Today programme's science correspondent Tom Feilden, explains that this is the question now driving a new generation of astronomers and the development of new astronomical instruments and telescopes.
First broadcast on BBC Radio 4's Today programme on Friday 15 March 2013.